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Thread: latest Project - 220 WPC Amp

  1. #1
    Senior Member HipoFutura's Avatar
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    latest Project - 220 WPC Amp

    Started this months ago, but recently made some progress. The boards are done, and I roughed out the case today. Still need beau coupe work to make it pretty, but all in time. Hopefully, in a couple of weeks I'll have it running. I'm going to build two; one for each 2235 sub. The boards are designed and copy righted by Anthony Holton (Aussie Amps). It's 220W RMS at 8 ohms per channel. Don't know what the wattage is bridged, but it will be a scarry number!

    Don
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    Senior Member grumpy's Avatar
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    Nice If current capability isn't a limitation, that would make for nearly
    900w into 8 ohms. Gonna need one heck of a PS to keep up with that.

    -grumpy

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    Senior Member HipoFutura's Avatar
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    Grumpy, the power supply is an Avel 625VA (2 - 50VAC secondaries) toroidal tranny with 40,000uf per rail. I'm building a surge board that will slow start the amp. This will keep it from tripping the house circuit breaker.

    Don

  4. #4
    Senior Member grumpy's Avatar
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    running the secondaries in parallel? (guess I can look up Aussie Amps)

    -grumpy

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    Senior Member HipoFutura's Avatar
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    The PSU is symmetrical with +/- 50V rails.

    Don

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    Senior Member grumpy's Avatar
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    PSU

    Hi Don,

    Sounds like that will get you maybe 140W at 8 ohms, unbridged, if I understand correctly that your DC rail voltages will be +/- 50V ==> (<35Vrms**2)/8. 10V higher rails would
    bump you up closer to 220W (if my coffee deprived brain is working this morning)
    Cool project, regardless.

    -grumpy

  7. #7
    Senior Member HipoFutura's Avatar
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    Grumpy, I hate to admit it, but you're in over my head. I need a bigger brain! Holton must be using 42V (peak to peak) - based on the 50V rails - for his 220W calculation. Why did you use 35V?

    Don

  8. #8
    Senior Member grumpy's Avatar
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    I need a bigger brain!

    Me too.

    We're talking RMS watts (a way to express AC power), so it's not a straight conversion
    using the p-p AC voltage squared over the resistance. One way to think of Wrms is as an equivalent
    measure for how hot a load resistor would get if you put an equivalent amount of DC watts through it.
    Since AC isn't "full on" all the time, to get an equivalent amount of "heat" into the load,
    you have to introduce a factor. This is the peak-to-peak voltage (100V max, in your case)
    divided by two (think of it so far as a rectified sine wave...the resistor doesn't care
    what direction the current is flowing in... then adjust for the "dips" between the bumps (now swinging from 0 to +50V) to
    get an equivalent power value (multiply by .707), which is the average value over time.

    So your PSU is capable of supporting an output voltage of 35Vrms (or a bit less, if you include normal losses).
    Now you can use V**2/R to get Wrms... something less than 150Wrms.

    42Vrms would require +/-60V rails. The amp's output waveform has to stay within the
    confines of your PS rails (ignoring output transformers & other weird stuff).

    Hope I didn't make things worse. A picture would help (if I had one handy).

    -grumpy

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    Senior Member HipoFutura's Avatar
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    The '.707' multiplier was the piece I was missing. Now I get it. I'll email Holtan and see what he has to say.

    Don

  10. #10
    Senior Member grumpy's Avatar
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    checked out web site...

    Ah... 50VAC secondary windings are assumed to be able to support something
    like +/- 65V(+) dc rails. This may be what was missing. Makes sense now. -grumpy

    (50Vrms transformer output gets you ~70V peak, rectify this... hold the peak values
    with the humungo cap bank and you have your 65V PS rails)

  11. #11
    Senior Member HipoFutura's Avatar
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    Grumpy, Mr. Holton responded to my request regarding wattage. Since I can't cut/paste on this forum I'll have to paraphrase.

    The 50V rails produce 70VDC after conversion and filtering. Full power at 8 ohms drops it to 66.6V. With your formula it works out to 221W RMS. This also accounts for a 6V drop across the output stage.

    Well, there you have it! If I can find the time to finish up the case I should be able to fire this thing up in a few days. With it now looking like an amp I'm inspired to get it running sooner rather than later.

    I'll use this and its twin to replace the Phase Linear 400 Series Two currently driving the subs.

    Don

  12. #12
    Senior Member Rudy Kleimann's Avatar
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    625VA is not 900W RMS!

    625VA is approximately 625 Watts, but not really even that. You have to factor in the current-limiting introduced by the inductive reactance of the transformer coil windings. look at the back of audio gear: it will often state its power consumption in watts and Volt-Amps. The watts figure is usually around 20% less than the volt-amp figure. And all the watts of heat generated by the equipment is power consumed just running itself, and less to go to the speakers.

    You may get 900W peaks out of it, but for what duration those peaks can last, it's hard to say. Depends on how the transformer acts as the output transistors drain the electrons out of those caps.

    By its very nature, audio is extremely dynamic, so you shouldn't worry too much. However, there is a curve of power output vs. time period that will dictate the amplifier performance (and its sound) on high-power transients.

    The +/-50V transformer outputs were, as I expected, RMS voltage ratings, which is the norm. That means 70V peak (RMS * 1.414, aka square root of 2, which is the same thing Grumpy is saying, only taking the equation in reverse). Your AC outlet is 120V RMS too, or about 170 Volts peak.

    Don't worry about a soft-start circuit on those amps, unless the transformer or amp mfg. suggests the need for it, or your 120V source is already loaded heavily and you turn everything on at once. A 120V 20Amp circuit breaker will deliver 2400 watts continuous all day long without tripping, and your initial inrush current to those amps only lasts a few milliseconds.

    BTW- how you like those 2405's on the D-75?

  13. #13
    Senior Member grumpy's Avatar
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    625VA is not 900W RMS!
    Good points.
    I don't think anyone was seriously expecting a sustained 900w from this amp.
    (sorry Don, I've hijacked enough of this thread) -grumpy

  14. #14
    Senior Member HipoFutura's Avatar
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    Rudy, Grumpy has it right. I have yet to run my system over half volume. 200W RMS would actually be more than enough. But you know, bigger is better! If a few watts are good, then a whole bunch should be even better.

    The Crown D-75 is great with the 2405s. I'm very happy with it. However, I can still hear a faint hiss when I get within three feet of the tweets (at head level). It's constant and doesn't change pitch or volume. Gone as soon as the music starts or you pull your head away from the speakers.

    I have way too much hardware for a home stereo system. It's a sickness, but having bunches of fun with it!

    Don

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    Very often the difference in watts and volt amps on a transformer lable is only theoretical assuming a power factor of .8 . The amps will be constrained by the winding size the volts is assuming a certain input and for some reason .8 seams to usually be the assumed powerfactor. However the core can (I think) actually limit the power so if they skimped on the core the watts may be the limit. actually I haven't seen very many transformers with a watt rating. Also are you talking rms for a minute or two or for 4 hours. If any body out there is a transformer engineer and my facts are wrong then just say so and maybe they won't be next time but I think that's right. Watts will never be more than volt amps.

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